Exhaust nozzle

ABSTRACT

There is disclosed an exhaust nozzle for a gas turbine engine, the exhaust nozzle comprising a frame extending along a longitudinal axis. The exhaust nozzle comprises a convergent petal pivotably attached at a convergent pivot point to the frame and extending axially downstream and radially inward from the frame, a follower roller fixed to the convergent petal on a radially outer side of the convergent petal, and a cam defining a working surface configured to engage the follower roller to react a force from the convergent petal. The cam is movable along a travel in an axial direction to actuate radial movement of the follower roller to pivot the convergent petal. The cam defines a concave working surface such that a contact angle between the follower roller and the cam varies along the travel to thereby vary a radial component of the force reacted by the cam.

CROSS-REFERENCE TO RELATED APPLICATIONS

This disclosure is based upon and claims the benefit of UK PatentApplication No. GB 1915793.2, filed on 31 Oct. 2019, the entire contentswhich are hereby incorporated herein by reference.

BACKGROUND Technical Field

The present disclosure relates to an exhaust nozzle for a gas turbineengine, and a gas turbine engine comprising the exhaust nozzle.

Description of the Related Art

Gas turbine engines may use a variable geometry convergent-divergent(con-di) exhaust nozzle to maximise the production of thrust. A typicalexhaust nozzle comprises a plurality of convergent petals which can bepivoted to converge, to reduce the size of an area for air flowexhausting from the engine.

SUMMARY

According to a first aspect of the disclosure, there is provided anexhaust nozzle for a gas turbine engine, the exhaust nozzle comprising aframe extending along a longitudinal axis and the exhaust nozzlecomprising: a convergent petal pivotably attached at a convergent pivotpoint to the frame and extending axially downstream and radially inwardfrom the frame; a follower roller fixed to the convergent petal on aradially outer side of the convergent petal; and a cam defining aworking surface configured to engage the follower roller to react aforce from the petal; wherein the cam is movable along a travel in anaxial direction to actuate radial movement of the follower roller topivot the convergent petal, whereby the cam defines a concave workingsurface such that a contact angle between the follower roller and thecam varies along the travel to thereby vary a radial component of theforce reacted by the cam.

A concave cam is intended to mean a cam having a curved profile, wherethe centre of curvature is radially inward in the nozzle from the cam.

The exhaust nozzle may comprise a divergent petal pivotably attached ata divergent pivot point to a downstream end of the convergent petal, thedivergent petal extending axially downstream and radially outward fromthe divergent pivot point.

The divergent petal may be connected to the frame by a linkage such thatthe frame, convergent petal, divergent petal and linkage form a four-barlinkage.

The linkage may be a thrust linkage which is actuatable to changelength.

The convergent petal may define a chord length from the convergent pivotpoint to the divergent pivot point. The roller may be fixed between40-80% along the chord length of the convergent petal from theconvergent pivot point.

The cam may be moveable between a contracted position in which the camis in a furthest upstream position and an expanded position in which thecam is in a furthest downstream position. The cam may be configured sothat a contact angle between the cam and the follower roller in theexpanded position is between 80-100 degrees from the longitudinal axis,preferably between 85-95 degrees from the longitudinal axis.

A ratio of a radius of the follower roller to an average radius ofcurvature of the cam may be 0.05 or above. A ratio of a radius of thefollower roller to a maximum radius of curvature of the cam may be 0.2or below.

The exhaust nozzle may comprise a plurality of convergent petalsangularly distributed around the exhaust nozzle, each comprisingrespective rollers, and the exhaust nozzle comprising a correspondingplurality of cams circumferentially spaced around the exhaust nozzle andconfigured to maintain engagement with a respective roller.

The cam may be fixed to a unison ring. Axial movement of the cam may beactuated by axial movement of the unison ring.

There may be a plurality of divergent petals corresponding to theplurality of convergent petals.

According to a second aspect of the disclosure, there is provided a gasturbine engine comprising an exhaust nozzle in accordance with the firstaspect.

The skilled person will appreciate that except where mutually exclusive,a feature described in relation to any one of the above aspects may beapplied mutatis mutandis to any other aspect. Furthermore, except wheremutually exclusive any feature described herein may be applied to anyaspect and/or combined with any other feature described herein.

DESCRIPTION OF THE DRAWINGS

Embodiments will now be described by way of example only, with referenceto the Figures, in which:

FIG. 1 is a sectional side view of a gas turbine engine;

FIG. 2 schematically shows a longitudinal cross-section of an exampleexhaust nozzle in a contracted configuration; and

FIG. 3 schematically shows a longitudinal cross-section of the exhaustnozzle in FIG. 2 in an expanded configuration.

DETAILED DESCRIPTION

With reference to FIG. 1, a gas turbine engine is generally indicated at10, having a principal and rotational axis 11. The engine 10 comprises,in axial flow series, an air intake 12, a propulsive fan 13, anintermediate pressure compressor 14, a high-pressure compressor 15,combustion equipment 16, a high-pressure turbine 17, an intermediatepressure turbine 18, a low-pressure turbine 19 and an exhaust nozzle 20.A nacelle 21 generally surrounds the engine 10 and defines both theintake 12 and the exhaust nozzle 20.

The gas turbine engine 10 works in the conventional manner so that airentering the intake 12 is accelerated by the fan 13 to produce two airflows: a first air flow into the intermediate pressure compressor 14 anda second air flow which passes through a bypass duct 22 to providepropulsive thrust. The intermediate pressure compressor 14 compressesthe air flow directed into it before delivering that air to thehigh-pressure compressor 15 where further compression takes place.

The compressed air exhausted from the high-pressure compressor 15 isdirected into the combustion equipment 16 where it is mixed with fueland the mixture combusted. The resultant hot combustion products thenexpand through, and thereby drive the high, intermediate andlow-pressure turbines 17, 18, 19 before being exhausted through thenozzle 20 to provide additional propulsive thrust. The high 17,intermediate 18 and low 19 pressure turbines drive respectively thehigh-pressure compressor 15, intermediate pressure compressor 14 and fan13, each by suitable interconnecting shaft.

Other gas turbine engines to which the present disclosure may be appliedmay have alternative configurations. By way of example such engines mayhave an alternative number of interconnecting shafts (e.g. two) and/oran alternative number of compressors and/or turbines. Further the enginemay comprise a gearbox provided in the drive train from a turbine to acompressor and/or fan.

FIG. 2 shows an example exhaust nozzle 20 having a variableconverging-diverging (con-di) exhaust in a contracted configuration. Inother examples, the exhaust nozzle may only have a variable convergingexhaust, without a divergent section.

The nozzle 20 comprises a radially outer support frame 30 extending froman upstream end (left side in the Figures) to a downstream end (rightside in the Figures) along a longitudinal axis 50, which is coaxial withthe rotational axis 11 of the gas turbine engine 10 described withreference to FIG. 1, when the nozzle 20 is mounted in the gas turbineengine 10. The frame 30 comprises an annular portion 30 a which iscoaxial with the longitudinal axis 50 and which converges from theupstream end to the downstream end (i.e. the radius of the annularportion 30 a reduces in a downstream direction along the longitudinalaxis 50).

The nozzle 20 comprises a plurality of convergent petals 32 which areangularly distributed about the longitudinal axis 50 within the nozzle20 at an upstream end of the nozzle 20, and a corresponding plurality ofdivergent petals 34 which are angularly distributed about thelongitudinal axis 50 within the nozzle 20 downstream of (and connectedto) the plurality of convergent petals 32. The plurality of convergentpetals 32 and divergent petals 34 are configured to provide a convergingand then diverging cross-sectional area for air flow exhausting from thegas turbine engine 10, for example to choke the flow and achievesupersonic exit velocities in the divergent section.

The extent of convergence of the convergent petal 32 and the extent ofdivergence of the divergent petal 34 is variable, as will be explainedin detail below. In the contracted configuration, the convergent petal32 is in a contracted position in which it is at a maximum convergence(i.e. it reduces the air flow area to a minimum along the longitudinalaxis 50). In this example, the convergent petal 32 in the contractedposition is angled radially inwardly at an angle of approximately 40degrees with respect to the longitudinal axis 50 (with the frame ofreference being such that 0 degrees would correspond to the convergentpetal 32 being parallel with the longitudinal axis 50). In otherexamples, the angle of the convergent petal with respect to thelongitudinal axis in the contracted position may be less than 40degrees, such as 35 degrees or 30 degrees.

The configuration of each convergent petal 32 and respective divergentpetal 34 is identical, and as such it will be described below withrespect to a single convergent petal 32 and respective divergent petal34.

The frame 30 comprises a first extension 30 b extending radially inwardsfrom the annular portion 30 a of the frame 30 at an upstream end of theframe 30. The convergent petal 32 is pivotably attached to the firstextension 30 b at a convergent pivot point 36. The convergent petal 32extends axially downstream and radially inwardly from the frame 30 andfrom the convergent pivot point 36, in the contracted position.

The divergent petal 34 is pivotably attached to a downstream end of theconvergent petal 32, at a divergent pivot point 38. The divergent petal34 is pivotably attached to a linkage 40 at a point on the divergentpetal 34 downstream of the divergent pivot point 38. The linkage 40 ispivotably attached to a second extension 30 c which extends radiallyinwardly from the annular portion 30 a of the frame 30, at a location onthe annular portion 30 a downstream of the first extension 30 b. Thedivergent petal 34 is connected to the linkage 40 and convergent petal32 such that it extends axially downstream and radially outwardly formthe divergent pivot point 38.

The frame 30, convergent petal 32, divergent petal 34 and linkage 40therefore form a four-bar linkage, such that pivoting movement of theconvergent petal 32 induces predictable pivoting movement of thedivergent petal 34. Whilst all members of the four-bar linkage are ofconstant length, the four-bar linkage is said to have one degree offreedom (i.e. such that for each angular position of the convergentpetal 32 there is a single corresponding angular position of thedivergent petal 34).

The linkage 40 in this example is a thrust linkage comprising atelescopic extension. The thrust linkage 40 is actuatable to change inlength, so that the pivoting movement of the divergent petal 34 inresponse to pivoting movement of the convergent petal 32 can beadjusted, thereby providing a second degree of freedom in the four-barlinkage.

The nozzle 20 further comprises a unison ring 44 disposed radiallyoutwardly of the convergent pivot point 36. The unison ring 44 isannular and extends around the longitudinal axis 50 within the annularportion 30 a of the frame 30. A cam 46 is fixedly attached to the unisonring 44 and extends axially downstream and radially inwards from theunison ring 44, such that it is disposed radially outwardly from theconvergent petal 32. Since the cam 46 is fixedly attached to the unisonring 44, it cannot translate or rotate with respect to the unison ring44. The cam 46 defines a curved, concave working surface 48 on aradially inner side of the cam 46 (i.e. the radius of curvature of thecam 46 is located radially inward of the cam 46 within the nozzle 20,with respect to the longitudinal axis 50).

A follower roller 42 is fixed to the convergent petal 32 on a radiallyouter side of the convergent petal 32 (i.e. with respect to thelongitudinal axis 50). The working surface 48 of the cam 46 isconfigured to engage the follower roller 42, such that contact ismaintained between the cam 46 and the follower roller 42. It will beappreciated that in use, aerodynamic forces act on the convergent petal32 so that the follower roller 42 is urged against the cam 46, asexplained in further detail below.

The convergent petal 32 defines a chord length along the convergentpetal 32 from the convergent pivot point 36 to the divergent pivot point38. The follower roller 42 in this example is fixed at 50% of the chordlength from the convergent pivot point 36. In other examples, thefollower roller may be fixed to the convergent petal at any suitablelocation, for example between 40-80% of the chord length from theconvergent pivot point.

During use, air flow through the exhaust nozzle is directed radiallyinward by the convergent petal 32, and permitted to flow radiallyoutward past the divergent petal 34. Therefore, the air flow exerts aforce on the convergent petal 32 as it is directed radially inward. Thisforce is transferred through the follower roller 42 to the cam 46 whichreacts the force from the convergent petal 32.

The cam 46 is moveable along a travel in an axial direction. Movement ofthe cam 46 in the axial direction actuates radial movement of thefollower roller 42, because the cam 46 and follower roller 42 areengaged. Radial movement of the follower roller 42 pivots the convergentpetal 32 about the convergent pivot point 36. Therefore, the pivotingangle of the convergent petal 32 with respect to the longitudinal axis50 can be controlled by axial movement of the cam 46.

As explained above, the pivoting of the convergent petal 32 inducespivoting of the divergent petal 34 about the divergent pivot point 38due to the four-bar linkage arrangement.

In this example, there are a plurality of cams 46 such that there is acam 46 for every convergent petal 32 around the nozzle 20. In otherexamples, there may be fewer cams, such as one cam for every twoconvergent petals. The convergent petals for which there is nocorresponding cam may then be coupled to an adjacent convergent petalwhich does have a corresponding cam, such that pivoting movement of theadjacent convergent petal induces identical movement of the convergentpetal without a corresponding cam.

The unison ring 44, which is fixedly attached to the cam 46, is moveablein an axial direction. Around the circumference of the nozzle 20, theplurality of cams 46 are fixed to the unison ring 44 such that axialmovement of the unison ring actuates corresponding axial movement ofeach of the plurality of cams 46. Therefore, each of the plurality ofconvergent petals 32 can be actuated to pivot by a corresponding amountby axially moving the unison ring 44, and the amounts may besubstantially identical with sufficient control of manufacturingtolerances.

Axial movement of the unison ring 44 is controlled by a plurality ofactuators 52. In this example, there are four actuators distributedaround and within the nozzle 20 (only two are shown). In other examples,there may be any suitable number of actuators distributed around thenozzle to move the unison ring axially. The actuators 52 in this exampleare in the form of telescopic cylinders, which can move axially betweena fully retracted position, and a fully extended position.

As explained above, FIG. 2 shows the nozzle 20 in a contractedconfiguration. In the contracted configuration, the actuator 52 is in afully retracted position, such that the unison ring 44 and cam 46 are ina furthest upstream position. A downstream part of the cam 46 istherefore engaged, and in contact with the follower roller 42, such thatthe convergent petal 32 is in the contracted position. It will beappreciated that the total force exerted on the convergent petal 32 willtend to be largest in use (for given turbine exit flow conditions) whenthe convergent petal 32 is in the contracted position, because it is ata maximum convergence corresponding to a maximum change in direction ofthe air flow through the nozzle 20.

FIG. 3 shows the example exhaust nozzle 20 of FIG. 2 in an expandedconfiguration. In the expanded configuration, the convergent petal 32 isin an expanded position, in which it is pivoted furthest radiallyoutward. The convergent petal 32 in the expanded position is angledradially inwardly at an angle of approximately 10 degrees with respectto the longitudinal axis 50. In other examples, the angle of theconvergent petal with respect to the longitudinal axis in the expandedposition may be more than 10 degrees, such as at least 15 degrees or atleast 20 degrees.

In the expanded configuration, the actuator 52 is fully extended so thatthe unison ring 44 and cam 46 are in a furthest downstream position. Anupstream part of the cam 46 is therefore engaged, and in contact withthe follower roller 42, such that the convergent petal 32 is in theexpanded position.

Axial movement of the cam 46 is therefore controlled by extension andretraction of the actuators 52, and the cam 46 is moveable along atravel between the contracted configuration (FIG. 2) and the expandedconfiguration (FIG. 3), in which the cam 46 is in a respectivecontracted position and expanded position.

The concave working surface 48 of the cam 46 ensures that a contactangle between the follower roller 42 and the cam 46 varies along thetravel to thereby vary a radial component of the force from theconvergent petal 32 reacted by the cam 46.

Referring back to FIG. 2, a contact angle of a contact line 54 betweenthe cam 46 and the follower roller 42 in the contracted configuration isapproximately 45 degrees with respect to the longitudinal axis 50. Thisis lower than in previously considered systems due to the concaveprofile of the working surface 48 of the cam 46. This reduces the radialload transferred to the cam 46 and the unison ring 44 in the contractedconfiguration compared to previously considered systems.

Since the convergent petal 32 experiences the highest forces in thecontracted position, the lower contact angle between the cam 46 andfollower roller 42 in the contracted configuration reduces the maximumradial load transferred to the cam 46, and to the unison ring 44 in use.This enables a weight saving, as the cams 46 and the unison ring 44 canbe made more lightweight. This is particularly advantageous in examplesin which there is no unison ring, for example for a 2D non-axisymmetricnozzle, as the radial load cannot be constrained in a hoop continuouscomponent, such that the radial loads must be reduced for the sameweight nozzle.

As can be seen in FIG. 3, the concave working surface 48 of the cam 46is configured so that a contact angle of a contact line 56 between thecam 46 and the follower roller 42 in the expanded position isapproximately 90 degrees with respect to the longitudinal axis 50. Inother examples, the contact angle in the expanded position may bebetween 80 to 100 degrees. This ensures that the axial load which istransferred to the cam 46, and therefore to the unison ring 44 from theconvergent petal 32, is low or tends to zero in the expandedconfiguration.

Further, fixing the follower roller 42 to the convergent petal 32between 40-80% along the chord length of the convergent petal 32 reducesthe radial load on the unison ring 44 in the convergent position becausea moment arm between the convergent pivot point 36 and the contact pointbetween the cam 46 and the follower roller 42 is increased compared withpreviously considered arrangements. A moment about the convergent pivotpoint 36 from the pressure of the gas on the convergent petal 32 must bereacted by the cam 46 at the contact point between the cam 46 and thefollower roller 42. The reacting moment of the cam 46 is thereforeachieved with a lower reaction force if the moment arm is increased, andthe reacting moment of the cam 46 is achieved with a higher reactionforce if the moment arm is reduced.

The applicant has found that mounting the follower roller 42 at aposition less than 40% along the chord length of the convergent petal 32from the convergent pivot point 36 results in a rapid increase in radialload on the unison ring 44 and actuator 52 in the contractedconfiguration due to the decreasing moment arm. In contrast, althoughthe radial load reduces as the distance of mounting the follower roller42 from the convergent pivot point 36 increases, the applicant has foundthat mounting it further than 80% along the chord length from theconvergent pivot point results in clashes between the follower rollerand surrounding components in use.

In this example, a radial ratio of a radius of the follower roller tothe radius of curvature of the cam is approximately 0.1. In otherexamples, the ratio may be between 0.05 and 0.2. The applicant has foundthat a ratio over 0.2 results in a rapidly decreasing contact anglebetween the cam 46 and the follower roller 42 with respect to thelongitudinal axis, such that the axial load transferred to the unisonring 44 and actuators 52 rapidly increases towards the contractedconfiguration. The applicant has also found that a ratio of below 0.05results in the cam 46 being too long in a typical nozzle such that itwould collide with the divergent petal 34 when moving towards theexpanded configuration. The radius of curvature in this example is theradius of curvature in a plane intersecting the longitudinal axis 50,which may be the engine centreline axis 11.

In some examples, the radius of curvature of the cam may not beconstant. The radius of curvature used in such examples to calculate thecomparable radial ratio is the average radius of curvature along curvedportions of the cam for lower limit of 0.05 and the maximum radius ofcurvature along curved portions of the cam for the upper limit of 0.2.

Although it has been described that the frame comprises discreteradially inward extending extensions to which the convergent petal andlinkage are coupled, the extensions may be in the form of annularextensions, which are continuous around the circumference of the nozzleand frame, or any suitable support structure.

It will be understood that the invention is not limited to theembodiments above-described and various modifications and improvementscan be made without departing from the concepts described herein. Exceptwhere mutually exclusive, any of the features may be employed separatelyor in combination with any other features and the disclosure extends toand includes all combinations and sub-combinations of one or morefeatures described herein.

What is claimed is:
 1. An exhaust nozzle for a gas turbine engine, theexhaust nozzle comprising a frame extending along a longitudinal axisand the exhaust nozzle comprising: a convergent petal pivotably attachedat a convergent pivot point to the frame and extending axiallydownstream and radially inward from the frame; a follower roller fixedto the convergent petal on a radially outer side of the convergentpetal; and a cam defining a working surface configured to engage thefollower roller to react a force from the convergent petal; wherein thecam is movable along a travel in an axial direction to actuate radialmovement of the follower roller to pivot the convergent petal, wherebythe cam defines a concave working surface such that a contact anglebetween the follower roller and the cam varies along the travel tothereby vary a radial component of the force reacted by the cam.
 2. Theexhaust nozzle according to claim 1, comprising a divergent petalpivotably attached at a divergent pivot point to a downstream end of theconvergent petal, the divergent petal extending axially downstream andradially outward from the divergent pivot point.
 3. The exhaust nozzleaccording to claim 2, wherein the divergent petal is connected to theframe by a linkage such that the frame, convergent petal, divergentpetal and linkage form a four-bar linkage.
 4. The exhaust nozzleaccording to claim 3, wherein the linkage is a thrust linkage which isactuatable to change length.
 5. The exhaust nozzle according to claim 2,wherein the convergent petal defines a chord length from the convergentpivot point to the divergent pivot point, and wherein the roller isfixed between 40-80% along the chord length of the convergent petal fromthe convergent pivot point.
 6. The exhaust nozzle according to claim 1,wherein the cam is moveable between a contracted position in which thecam is in a furthest upstream position and an expanded position in whichthe cam is in a furthest downstream position, and wherein the cam isconfigured so that a contact angle between the cam and the followerroller in the expanded position is between 80-100 degrees from thelongitudinal axis, preferably between 85-95 degrees from thelongitudinal axis.
 7. The exhaust nozzle according to claim 1, wherein aratio of a radius of the follower roller to an average radius ofcurvature of the cam is 0.05 or above.
 8. The exhaust nozzle accordingto claim 1, wherein a ratio of a radius of the follower roller to amaximum radius of curvature of the cam is 0.2 or below.
 9. The exhaustnozzle according to claim 1, comprising a plurality of convergent petalsangularly distributed around the exhaust nozzle, each comprisingrespective rollers, and the exhaust nozzle comprising a correspondingplurality of cams circumferentially spaced around the exhaust nozzle andconfigured to maintain engagement with a respective roller.
 10. Theexhaust nozzle according to claim 9, wherein the cam is fixed to aunison ring, and wherein axial movement of the cam is actuated by axialmovement of the unison ring.
 11. The exhaust nozzle according to claim9, wherein there are a plurality of divergent petals corresponding tothe plurality of convergent petals.
 12. A gas turbine engine comprisingan exhaust nozzle according to claim 1.